The present invention relates to the communication of digital television signals, and more particularly to a band-width efficient scheme for providing user data in a digital television data stream without the need to reserve space for the user data in advance. Examples of such user data include closed caption data, non-realtime video data (e.g., vertical interval test signalxe2x80x94VITS), sampled video data and automated measurement of lineup (AMOL) data.
Digital transmission of television signals can deliver video and audio services of much higher quality than analog techniques. Digital transmission schemes are particularly advantageous for signals that are broadcast via a cable television network or by satellite to cable television affiliates and/or directly to home satellite television receivers. It is expected that digital television transmitter and receiver systems will replace existing analog systems just as digital compact discs have replaced analog phonograph records in the audio industry.
One way to transmit the compressed video data to a receiver is in the form of packets contained within a packetized data stream. Typically, packets carrying compressed video data are multiplexed with other packets, e.g., carrying corresponding audio data and control information necessary to reconstruct a television signal. One standard for transporting digital television signals in this manner is the MPEG-2 standard, details of which can be found in the International Organisation for Standardisation, ISO/IEC 13818-1, International Standard, 13 Nov. 1994 entitled xe2x80x9cGeneric Coding of Moving Pictures and Associated Audio: Systems,xe2x80x9d recommendation H.222.0, incorporated herein by reference. Further details of the video syntax and semantics for MPEG-2 video can be found in International Organisation for Standardisation, ISO/IEC 13818-2, International Standard, 1995 entitled xe2x80x9cGeneric Coding of Moving Pictures and Associated Audio: Video,xe2x80x9d recommendation H.262, also incorporated herein by reference.
In the MPEG-2 system (and the similar DigiCipher(copyright) II system proprietary to General Instrument Corporation, the assignee hereof) a transport stream, or transport multiplex is made up of a contiguous set of fixed length packets. The video sequence is transported using a hierarchical structure in which a sequence header is followed by various extensions, user data, a group of pictures (xe2x80x9cGOPxe2x80x9d) header, optional user data, a picture header, etc. The sequence header provides information for a sequence of pictures, which in general will include more than one GOP. This information includes, for example, horizontal and vertical size values, aspect ratio, frame and bit rate, and quantization parameters for the video data. A user data extension can also be included which, among other things, provides additional data for use by decoders. The DigiCipher(copyright) II standard provides for the transport of additional user data after the sequence header, in order to identify a DigiCipher(copyright) II signal and the use of any special video compression techniques used within a sequence, including DigiCipher(copyright) special prediction and block motion estimation.
In both the MPEG-2 and DigiCipher(copyright) II syntaxes, a sequence display extension containing, e.g., video format and color description information, is provided in addition to the sequence extension and user data. A subsequent group of pictures header provides, among other information, a time code. Thereafter, a picture header is provided which includes various information pertaining to a corresponding picture in a sequence of pictures to be displayed. A picture extension and, ultimately, the actual picture data to be decoded and reproduced for viewing, is then provided. It is noted that MPEG does not specify the order in which various extensions (such as the sequence display extension) or the user data must be transmitted beyond the fact that they must be after the sequence extension and before the GOP header (if provided) or the picture header. MPEG does not require GOP headers to be sent, and such headers may be bypassed in particular implementations.
In a practical transmission system it may be necessary to include additional data at different times for specific purposes, such as providing closed captioning, VITS, auxiliary real time video, and AMOL data. Such additional data may be carried in the vertical blanking interval (VBI) portions of a television signal, and is referred to herein as xe2x80x9cVBI user information.xe2x80x9d Typically, the insertion of such VBI user information would require a transport syntax to allocate and reserve fixed length portions into which the VBI user information is inserted. If the VBI user information is not used, the fixed length portions would be unusable for other information and bandwidth would be wasted.
It would be advantageous to provide a transport syntax for digital television data that would accommodate various types of VBI user information which may or may not be used at any given time, without requiring space to be reserved for the information in advance. Such a scheme would enable the economical management of bandwidth while providing flexibility as to the transport of VBI user information. The present invention provides a transport method and apparatus enjoying the aforementioned advantages.
In accordance with the present invention, a bandwidth efficient method is provided for using a digital television data stream to transport variable amounts of different types of information carried in a vertical blanking interval (VBI) portion of a television signal. Information to be transported is extracted from portions of input video data that correspond to vertical blanking intervals of successive video frames. First identifiers are provided for the extracted information. The first identifiers specify a count of a particular type of information associated with the identifier. The first identifiers and the associated information are inserted into VBI related portions of the digital television data stream. The data stream, including the first identifiers and the information, is then communicated to a receiver. The first identifiers enable the adjustment of the data stream to accommodate variable amounts of different types of information without space being reserved for specific amounts and types of information in advance.
Second identifiers can be provided for the extracted information. The second identifiers designate a priority of the particular type of information specified by the first identifiers. In an illustrated embodiment, the first and second identifiers precede the information associated therewith in the data stream. Each second identifier follows a counterpart first identifier. Thus, the syntax provides a first identifier specifying a count of a particular type of information, followed by a second identifier specifying a priority of the information, followed by the information itself. It should be appreciated that each type of information (i.e., xe2x80x9cconstruct,xe2x80x9d the size of which is dictated by the count) may include a priority, such that a video picture can include constructs of various priorities.
The extracted information can comprise, for example, one or more of closed caption data, non-realtime video data, sampled video data and AMOL data, referred to herein as VBI user information because it is provided in the VBI portions of a television signal. When the extracted information comprises non-realtime video data, the syntax provides sequence numbers and segment numbers for portions of the non-realtime video data. When the extracted information comprises AMOL data, identifiers are provided to designate whether the AMOL data is null data represented in an abbreviated form. For example, where the AMOL data comprises a string of zeros, the string can be runlength coded to avoid the need to send the entire string of zeros, thereby saving bandwidth. The syntax can also provide identifiers to designate parity values for the AMOL data.
In the illustrated embodiment, the digital television data stream is assembled to provide a first identifier in the VBI portions specifying a count for closed caption data, followed by a first identifier specifying a count for non-realtime video data, followed by a first identifier specifying a count for sampled video data, followed by a first identifier specifying a count for AMOL data. It will be appreciated that the syntax formed in this order constitutes only one possible embodiment and that the ordering can be rearranged without departing from the concept of the present invention.
The present invention also provides a method for decoding information communicated in the manner set forth above. In such a decoding method, the digital television data stream is received at a decoder. As the VBI portions of the data stream are reached, the first identifiers are detected. Closed caption data is processed when the first identifier specifying the count of closed caption data indicates the presence of such data. Non-realtime video data is processed when the first identifier specifying the count of non-realtime video data indicates the presence of such data. Sampled video data is processed when the first identifier specifying the count of sampled video data indicates the presence of such data. AMOL data is processed when the first identifier specifying the count of AMOL data indicates the presence of such data.
Where second identifiers are provided in the VBI portions to designate a priority of the particular type of information specified by the first identifiers, the second identifiers are detected in the received data stream. The processing of particular information is inhibited if the priority thereof identified by the corresponding second identifier is not a priority supported by the decoder. In this manner, decoders having different capabilities can be deployed in the field for operation with a common data stream. Information that is not supported by a particular decoder will not adversely affect the operation of the decoder since the decoder will be informed by the priority identifier that it is to ignore the particular information.
Receiver apparatus is provided for decoding information contained in digital television signals. The receiver includes means for detecting information in a vertical blanking portion of a received digital television signal identifying a count of closed caption data, a count of non-realtime video data, a count of sampled video data, and a count of AMOL data that may be present in the vertical blanking portion. Means responsive to the detecting means process the closed caption data when the count of closed caption data indicates the presence of such data in the vertical blanking portion. Means responsive to the detecting means process non-realtime video data when the count of non-realtime video data indicates the presence of such data in the video blanking portion. Means responsive to the detecting means process sampled video data when the count of sampled video data indicates the presence of such data in the vertical blanking portion. Means responsive to the detecting means process the AMOL data when the count of AMOL data indicates the presence of such data in the vertical blanking portion.
The receiver apparatus can further comprise means for detecting information identifying a priority of the received data. Means are provided for inhibiting the processing of the data if the priority is not supported by the receiver apparatus. In an illustrated embodiment, the received digital television signal has an MPEG format.
Apparatus is provided for communicating information in a bandwidth efficient manner in a digital television data stream. Means are provided for extracting information from portions of digital video data that correspond to vertical blanking intervals of successive video frames. At least one particular type of information extracted by the extracting means is quantified and a first identifier is associated therewith. The first identifier specifies a count of the particular type of information following the identifier. Means are provided for combining the extracted information including the first identifiers associated therewith with active video portions of the digital video data to form the digital television data stream. The first identifiers enable the adjustment of the data stream to accommodate variable amounts and types of information without space being reserved for the information in advance.
The apparatus can further comprise means for associating second identifiers with the extracted information. The second identifiers designate a priority of the particular type of information specified by the first identifiers. In the illustrated embodiment, each of the second identifiers is inserted into the data stream following a counterpart first identifier. The first identifiers can provide counts for closed caption data, non-realtime video data, sampled video data and AMOL data.